The present invention is directed to recovery of hydrocarbons from residual feedstock by differential solution in a selective solvent. Examples of residual feedstocks are residual oils from distillation of petroleum as well as residua derived from tar sands and the destructive distillation or hydrogenation of coal.
The necessity for the economic recovery of hydrocarbon values from these residua is becoming ever important in light of the increased shortage of hydrocarbon reserves.
Solvent deasphalting as the general process is more commonly known, involves the separation of petroleum residua into an asphalt fraction which contains most or all of the very high molecular weight compounds, metal-containing compounds, and inorganic matter, and a deasphalted, normally paraffinic oil fraction which is relatively free of asphalt and metals.
Deasphalting is particularly useful in converting high-sulfur petroleum residue to low-sulfur fuel oil, since removal of the asphalt fraction makes the deasphalted oil amenable to catalytic hydrodesulfurization. If left in the residue these constituents will foul and deactivate the hydrodesulfurization catalyst.
In the practice of deasphalting, a low-molecular weight hydrocarbon typically an aliphatic hydrocarbon is mixed with the feedstock, resulting in precipitation of the asphalt fraction with a minor portion of the solvent, while the more soluble deasphalted oil is extracted as a low-density solution in the major portion of the solvent. Solvents used include among others, propane, isobutane, normal butane, pentane, hexane, and heptane, and mixtures thereof.
To achieve the desired separation it is necessary to employ a relatively large volume of solvent. The normal practice is to circulate from four to ten volumes of solvent to an extractor for each volume of feed. It is, therefore, necessary to evaporate and condense the solvent used in the deasphalting circuit. As a consequence large amounts of energy are needed for heating and cooling.
In a conventional practice of deasphalting, the solvent and residue feed are mixed in the extractor. The operation may be co-current or countercurrent. Extraction vessels may be packed or stirred. More than one extractor may be used in parallel or in series.
The extractor is operated at a moderately elevated temperature relative to the circulating temperature of the solvent, and at pressure sufficient to avoid vaporization of solvent.
When a deasphalted oil mix and asphalt mix leave the extractor, the deasphalted oil mix contains most of the solvent. The deasphalted oil mix is reduced in pressure by a pressure reduction valve, then heated in a heat exchanger to evaporate the solvent. The solvent vapors are separated from deasphalted oil in a flash drum. The pressure on the flash drum is selected for a subsequent condensing step. Liquid deasphalted oil is separated from the flash drum. Still containing a minor amount of solvent, the oil is steam stripped of residual solvent.
The asphalt mix from the extractor containing approximately equal volumes of solvent and asphalt is heated in an asphalt mix heater to a temperature suitable to strip out the solvent and reduce the viscosity of the asphalt portion to a workable range. The pressure is further reduced by a second expansion valve to favor evaporation of solvent. Vapors are separated in an asphalt flash drum and final traces of solvent steam stripped in an asphalt stripper.
Vapors from the flash drums are combined with the vapors from the strippers and condensed by heat exchange with a coolant usually water. When propane is the solvent, or part of the solvent mixture, it is convenient to operate the strippers at a lower pressure than the flash drums. In this case it is customary to compress the vapors removed from the strippers to return them into the common condensing system. Owing to restrictions imposed by cooling with water or air, the condensed solvent temperature is normally in the range from 80.degree. F to 140.degree. F.
Condensed solvent is accumulated in a solvent accumulator and recirculated to the extractor by a solvent pump.
The recirculated solvent is in these operations completely distilled by an external energy source in each circuit of the system. Since its volume is conventionally from four to ten times the volume of feed, considerable heat is required in a solvent heater, the deasphalted oil mix heater and the asphalt mix heater. It is customary to use steam as the heating medium for the solvent and deasphalted oil mix heaters, and a fired heater for the asphalt mix heater. In a typical case, the solvent heater requires almost half of the heat of the process.